Measuring microphones have been extensively used for measuring sound pressures. In many acoustic tests it has, however, been more expedient to measure the sound intensity, which is a vector size, rather than the sound pressure, which is scalar size. The sound intensity of a sound source is definable e.g. by integrating the sound intensity over a surface surrounding the sound source, in which no anechoic chamber will be needed. Even a strong background noise will not affect the measuring results detrimentally, as the sound sources outside the surrounding surface of integration do not contribute to the integrated sound intensity.
Intensity measuring is often based on a two-microphone-technique, e.g. as described by Finn Jacobsen in report No. 28, 1980, from the Technical University of Denmark, Acoustic Laboratory: "Measurements of Sound Intensity". Briefly, the method is based on the approximation that the intensity vector in a stationary sound field in a specific point is equal to the temporal average value of the product between the instantaneous pressure and the corresponding particle velocity in the same point, and that the vector of the particle velocity can be deduced from the sound pressure gradient in the point. This pressure gradient is approximately equal to the difference in pressure between two points located at a small distance from the reference point divided by the distance between the two measuring points.
A condition of the method being applicable in practice, and particularly at low frequencies, is that the two pressure microphones match extremely well, especially with respect to their phase characteristics, if measuring errors should be avoided.